In the state of the art, agricultural tractors are equipped with an internal combustion engine, which in operation drives the wheels for propelling the tractor, and a PTO shaft. The PTO shaft serves to drive auxiliary attachments, such as soil cultivating implements or baling presses. A PTO transmission is provided between the engine and the PTO output of the tractor. Such PTO transmissions generally have multiple transmission stages selectable by an operator, which for specific engine speeds lead to desired rotational speeds of the PTO shaft. The PTO transmission can also be brought into a neutral position, in which the PTO shaft can rotate freely.
In known transmissions the arrangements used are basically ones in which the PTO transmission is driven directly by a shaft, which rotates at the speed of the internal combustion engine and independently of the vehicle transmission. In other words, the rotational speed or the speed transmission ratio of this shaft for the PTO transmission is not influenced by the drive transmission. Provision might be made, however, for the shaft driven by the engine of the utility vehicle to be led to the PTO transmission through the housing of the drive transmission, without being influenced thereby. Consequently the PTO transmission and hence the PTO shaft or the PTO stub shaft will be driven at a speed which varies directly as a function of the speed of the engine of the utility vehicle. Adjustable speed-increasing and speed-reducing steps may also be provided here. Usual speeds of PTO shafts are either 540 or 1000 rpm.
A PTO transmission of this type may be driven by rotationally connecting the shaft, driven by the engine of the utility vehicle and serving to drive the PTO transmission, to a transmission input shaft of the drive transmission. The input shaft of the drive transmission can be isolated from the engine output shaft by means of a clutch, the shaft driven by the engine of the utility vehicle then also being isolated from the engine output shaft. In such an arrangement the PTO shaft can be operated only when the drive transmission is also being driven.
In another arrangement an additional clutch is provided, which serves to bring the shaft driven by the engine of the utility vehicle into rotational connection with the PTO transmission. With such an arrangement both the drive transmission and the PTO transmission may be driven simultaneously. It is also possible, however, to drive either the drive transmission or the PTO transmission separately.
If the PTO transmission or the PTO stub shaft of the PTO transmission is to rotate at a speed which varies as a function of the speed of travel of the utility vehicle (also referred to as a so-called road vehicle PTO), it is possible to couple the PTO transmission to a shaft serving to drive the rear axle of the utility vehicle.
It is not possible, especially in a transmission arrangement in which either the drive transmission or the PTO transmission can be driven separately, to couple a hydraulic pump and/or a transmission oil pump to an input shaft of the drive transmission, which is arranged on the engine output side, on the other side of the clutch for the drive transmission, since there is an operating state in which the drive transmission is inoperative. If the PTO transmission is in operation whilst this operating state prevails, it too should be supplied with transmission oil. Accordingly, such pumps are located directly on the engine of the utility vehicle and are driven directly by an engine output shaft.
Thus the possibility is known, of leading the shaft, which is driven by the engine of the utility vehicle and which serves to drive the PTO transmission, through the drive transmission and using a transmission shifting point to establish a rotational connection between the PTO transmission and the shaft driven by the engine of the utility vehicle on the one hand and the rear axle transmission output of the drive transmission on the other. This transmission shifting point also has a neutral position, in which the PTO transmission is isolated from the engine and from the drive transmission. This transmission shifting point is located in the drive transmission.
If the hydraulic pump and/or the transmission oil pump can or should no longer be arranged directly on the engine housing, it is feasible for the shaft, which is driven by the engine of the utility vehicle and which also serves to drive the PTO transmission, to be led through the drive transmission and to turn in constant rotational connection with the output shaft of the engine of the utility vehicle. The requisite rotational speed for the hydraulic pump and/or the transmission oil pump can then be harnessed from the constantly rotating shaft between the drive transmission and the PTO transmission. In order that the PTO transmission can now be switched on or off, a clutch which serves to establish a rotational connection to the shaft driven by the engine of the utility vehicle is provided on the input to the PTO transmission. In such a transmission arrangement it is not readily possible to achieve a road vehicle PTO, i.e. to couple the output speed of the PTO transmission to the speed of travel of the utility vehicle, since the hydraulic pump and/or the transmission oil pump is then operated at a speed varying as function of the speed of travel.